This invention relates generally to a color printing machine, and more particularly, cleaning brushes to remove toner additive film particle buildup on the photoconductive member.
In a colored image forming apparatus, an electrostatic latent image which is to be developed by a predetermined color is formed on a photoconductor by an optical system of a copying machine or printer. Then, the electrostatic latent image is developed by a developing unit which accommodates a predetermined colored toner to be used for development. The developed image after development is transferred onto a transfer material, intermediate transfer belt or the like on the photoconductor. Thereafter, an electrostatic latent image which is to be developed by the next predetermined color is formed on the photoconductor by an optical system. The electrostatic latent image thus formed is developed by a developing unit which accommodates the next predetermined colored toner to be used for development. The developed image thus obtained is transferred onto the transfer material, intermediate transfer belt or the like on the photoconductor by overlaying it on the image previously transferred. This toner image may be subsequently transferred to a support surface such as copy paper to which it may be permanently affixed by heating or by the application of pressure. After each transfer process, the toner remaining on the photoconductor is cleaned by a cleaning device.
However, when colored toners other than black toner are cleaned from the photoreceptor, there is a tendency for more residual toner to remain on the photoconductor. Thus, the photoreceptor is not able to be efficiently cleaned by the same process that is used to clean black toner alone from the photoreceptor. Possible reasons for the additional filming on the photoconductor caused by the color toners are the dye, pigment or additive used in the color toners. For example, zinc stearate (ZnSt) and Aerosil are essential additives to the color toners to enhance toner flow and stabilize developer conductivity. During the printing process the ZnSt is preferentially developed in the background regions of the photoreceptor, not transferred to the print paper, and subsequently smeared on the photoreceptor by the cleaner brushes. As the ZnSt film thickens with time, Aerosil particles become embedded in the film, causing a secondary print quality defect referred to as deletions, Charge Area Development (CAD) loss, or lateral charge conductivity. It is an objective of this invention to prevent smearing of the additive on the photoreceptor, remove additive film from the surface of the photoconductive member and to prevent the print quality defects caused by the embedded particles.
Certain print mode and/or material throughput conditions (i.e. throughput occurs when a high rate of material is being added and removed from developer housings; or when greater than 5% color area coverage occurs) in a single pass highlight color printer enable or promote photoreceptor filming by the Discharge Area Development (DAD) toner additive zinc stearate (ZnSt). Such film is the origin of the tri-level Image Push defect. Image Push defect is: (1) when the color image is moved going through the black housing. This is made worse when Zn St film is present and decreases the coefficient of friction between the toner and photoreceptor. (2) the movement of the color toner during the black development cycle due to the loss of coefficient of friction on the photoreceptor by the slippery Zn St film.
Various ideas as to how to improve cleaning efficiency have been disclosed. One publication suggested mixing toner with a small amount of low adhesive polymeric additive in smaller average particle size than that of the toner of each developer. Another publication discloses each developer being mixed with an abrasive for removing matter adhered to the photoconductor when the cleaning process is conducted. Yet another publication discloses an idea for removing a matter adhered to the photoconductor with a resin by providing a grinding device aside from a cleaning device.
However, in the colored image forming apparatus, it is a laborious task to mix the proper amount of suitable polymeric additive or abrasive with each developer and it eventually becomes expensive. Moreover, it is not preferable for use in forming a colored image which requires a delicate tone since it badly affects the clearness of color and permeability when the additive or abrasive are mixed with a colored toner other than black toner.
Various cleaning techniques have hereinbefore been used as illustrated by the following disclosures, which may be relevant to certain aspects of the present invention:
U.S. Pat. No. 4,945,388 to Tange et al. describes a method and apparatus for cleaning a color image from a photoreceptor wherein a black toner only image is transferred onto the photoreceptor periodically when the color developing units are actuated, without any transfer process, to remove residual black toner. A black toner only image is fixed to the photoreceptor during machine startup and after a certain number of copies.
Co-pending application, Ser. No. 07/56798 to Frankel et al., filed Aug. 20, 1990, describes an imaging device with a brush cleaner loaded with one type of toner to abrade the photoreceptor to remove the second type of toner.